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Full Idea
The only way we can discover that two things exist is by finding out that one has a quality not possessed by the other, or else that one has a relational characteristic that the other hasn't.
Gist of Idea
Two things can only be distinguished by a distinct property or a distinct relation
Source
Max Black (The Identity of Indiscernibles [1952], p.67)
Book Ref
'Metaphysics - An Anthology', ed/tr. Sosa,E. /Kim,J. [Blackwell 1999], p.67
A Reaction
At least this doesn't conflate relations with properties. Note that this idea is clearly epistemological, and in no way rules out the separateness of two objects which none of us can ever discern. Maybe the Earth has two Suns, which imperceptibly swap.